1. Field of the Invention
The present invention relates to an electrode, a battery precursor and a lithium battery.
2. Description of Related Art
Batteries are a type of electrochemical cell containing a pair of electrodes and an electrolyte disposed between the electrodes. One of the electrodes is called a cathode wherein an active material is reduced during discharge. The other electrode is called an anode wherein another active material is oxidized during discharge. Secondary batteries refer to batteries capable of charging electricity after discharge.
Recently, intensive research has been conducted on lithium secondary batteries because of their high voltage and high energy density. Lithium batteries refers to batteries having an anode containing an active material for releasing lithium ions during discharge. The active material may be metallic lithium and an intercalated material being capable of incorporating lithium between layers.
Particular attention has been paid to an electrode material for the cathode of the lithium secondary battery. For example, U.S. Pat. No. 4,833,048 discloses a cathode containing a disulfide compound for improving an energy density. This compound is represented by Rxe2x80x94Sxe2x80x94Sxe2x80x94R wherein R is an aliphatic or an aromatic organic group and S is a sulfur atom. An Sxe2x80x94S bond is cleaved by the electrolytic reduction in an electrolytic cell containing cation of M+ to form a salt represented by Rxe2x80x94Sxe2x88x92xc2x7M+. This salt returns to the Rxe2x80x94Sxe2x80x94Sxe2x80x94R by the electrolytic oxidation. U.S. Pat. No. 4,833,048 discloses a rechargeable battery obtained by combining a disulfide compound with metal M which supplies and captures the cations (M+). The rechargeable battery provides an improved energy density of at least 150 Wh/kg. The entire disclosure of U.S. Pat. No. 4,833,048 is incorporated herein as reference.
However, as the inventors of U.S. Pat. No. 4,833,048 reported in J. Electrochem. Soc., Vol. 136, No. 9, pp. 2570 to 2575 (1989), the difference between the oxidation potential and the reduction potential of the disulfide compound is very large. For example, when [(C2H5)2NCSSxe2x80x94]2 is electrolyzed, the oxidation potential differs from the reduction potential by 1 V or more. According to the theory of electrochemical reaction, the electron transfer of the disulfide compound proceeds extremely slowly at room temperature. Therefore, it is rather difficult to obtain a rechargeable battery providing a higher current output of 1 mA/cm2 or more at room temperature. The operation of a battery comprising an electrode of disulfide compound is limited to high temperatures in the range of 100xc2x0 to 200xc2x0 C., where the electron transfer can proceed faster.
U.S. Pat. No. 5,324,599 discloses a cathode for a lithium secondary battery containing the disulfide compound and a conductive polymer. The conductive polymer allows to operate the battery in much lower temperatures such as room temperature. The entire disclosure of U.S. Pat. No. 5,324,599 is incorporated herein as reference.
Japanese Patent No. 2,513,418, which corresponds to JP-A-5-175929, discloses a cathode containing carbon nanotubes. The carbon nanotubes are obtained by electric discharge between a pair of carbon rods. Japanese Patent No. 2,513,418 does not teach the disulfide compound. The entire disclosure of Japanese Patent No. 2513418 is incorporated herein as reference.
WO 95/07551 discloses an electrode, which may be used for a lithium secondary battery, containing carbon nanotubes. The carbon nanotubes are obtained by catalytic reactions. The document further discloses aggregates of carbon nanotubes disentangled by an ultrasonic homogenizer. The entire disclosure of WO 95/07551 is incorporated herein as reference.
According to the first aspect of the present invention, there is provided an electrode, comprising: an electrically conductive matrix containing a disulfide group, wherein an Sxe2x80x94S bond of the disulfide group is cleaved by electrochemical reduction and reformed by electrochemical oxidation; and a plurality of carbon nanotubes being dispersed in the electrically conductive matrix.
Preferably, the electrically conductive matrix may contain an electrically conductive polymer and an organic compound having the disulfide group. Alternatively, the electrically conductive matrix may contain an electrically conductive polymer having the mercapto group which is capable of forming disulfide group.
According to the second aspect of the present invention, there is provided a battery precursor, comprising: a cathode having: an electrically conductive matrix containing a disulfide group, wherein an Sxe2x80x94S bond of the disulfide group is cleaved by electrochemical reduction and reformed by electrochemical oxidation; and a plurality of carbon nanotubes being dispersed in the electrically conductive matrix; and a cathode current collector; wherein the cathode is coated onto the cathode current collector.
According to the third aspect of the present invention, a lithium battery, comprising: a cathode having: an electrically conductive matrix containing a disulfide group, wherein an Sxe2x80x94S bond of the disulfide group is cleaved by electrochemical reduction and reformed by electrochemical oxidation; and a plurality of carbon nanotubes being dispersed in the electrically conductive matrix; an anode having an active material for releasing lithium ions; and an electrolyte being disposed between the cathode and the anode.